Washington: The Republican establishment has intensified efforts to stop Donald Trump from winning the party nomination, with its 2012 presidential candidate Mitt Romney calling for a strategic vote against him only to have the billionaire firing back.

File image. Reuters

Romney, who unsuccessfully challenged President Barack Obama four years back, wrote on Friday in a Facebook post that he plans to vote on Tuesday for Trump's closest rival, Texas senator Ted Cruz, in party caucuses in Utah, where he lives.

He also called on Ohio governor John Kasich, for whom he campaigned before the Ohio primary this week, to step aside to give Republicans a better chance to block Trump's bid for the nomination.

"Today, there is a contest between Trumpism and Republicanism," Romney wrote. "Through the calculated statements of its leader, Trumpism has become associated with racism, misogyny, bigotry, xenophobia, vulgarity and, most recently, threats and violence. I am repulsed by each and every one of these."

"The only path that remains to nominate a Republican rather than Trump is to have an open convention. At this stage, the only way we can reach an open convention is for Senator Cruz to be successful in as many of the remaining nominating elections as possible," he wrote.

There is no love lost between the Republican establishment and Cruz, who once called Republican Senate majority leader Mitch McConnell a liar on the Senate floor, but apparently it now looks at him as a known devil and lesser of an evil than Trump.

Reacting to Romney's post, Trump took to Twitter to mock the former former Massachusetts governor, and Cruz. Asserting that Romney's support for Cruz was good for his campaign, Trump tweeted: "Failed presidential candidate Mitt Romney, the man who 'choked' and let us all down, is now endorsing Lyin' Ted Cruz. This is good for me!"

"Going to Salt Lake City, Utah, for a big rally. Lyin' Ted Cruz should not be allowed to win there — Mormons don't like LIARS! I beat Hillary," he added.

Later, appealing to the state's majority Mormon population at the rally, he even questioned whether Romney truly represented their faith. "Do I love the Mormons? I have many friends that live in Salt Lake City, and by the way, Mitt Romney is not one of them," Trump said to applause from his supporters. "Are you sure he's a Mormon? Are we sure?"

Cruz, who dubbed Romney as a seriously flawed messenger against Obama in 2012, and characterised him as part of the Republican Party's "mushy middle" quickly thanked him for the support in a tweet on Friday afternoon.

Meanwhile, conservative news channel Fox News too joined the war against Trump, alleging the real estate mogul has an "extreme, sick obsession" with its anchor Megyn Kelly. "Donald Trump's vitriolic attacks against Megyn Kelly and his extreme, sick obsession with her is beneath the dignity of a presidential candidate who wants to occupy the highest office in the land," the network said in its statement.

The sharply worded response came on Friday evening after Trump took his attacks against Kelly to a new level, referring to her as "sick", "crazy" and "overrated". Trump also called on his supporters to boycott her Fox News show, saying she was obsessed with criticising him.

Trump has been having a running feud with Kelly since last August, when he complained that she had treated him unfairly during the first Republican presidential debate. But it was unclear what precipitated his latest attack.